Any discussion of the prior art throughout the specification should in no way be considered as an admission that such prior art is widely known or forms part of the common general knowledge in the field.
Drinking water contains different contaminants such as particulate matter, harmful chemicals, bacteria, cysts and viruses. It is recommended by health professionals that such contaminants should be removed so that water becomes fit for consumption.
Activated Carbon is useful for the purification of drinking water as it aids in the removal of chemical contaminants, cysts, particulate matter and bacteria. However, activated Carbon particles have rough and porous texture which is capable of providing a favorable and protected surface for bacterial growth. As a result, algae and bacteria colonize the Carbon surface, thus diminishing the surface available to contact the water as well as allowing these organisms to grow. Even if the water is chlorinated, some bacteria are chlorine resistant. Thus, the activated Carbon actually can promote the growth of bacteria that would not grow in the chlorinated water per-se. The most predominant bacteria that were isolated from such water belonged to the genus Pseudomonas. In addition, bacteria of the genus Bacillus were also found in large quantities.
A method of making bound Carbon block filters for use in water purification has been described in U.S. Pat. No. 4,753,728 (Amway, 1988) in which Carbon particles are bonded into a filter block by a polymeric material having melt index of less than 1 gram per 10 minutes as determined by ASTM (American Society for Testing and Materials) D1238 standard, at 190° C., and 15 kg load.
CA2396510 (TYK Corp, 2003) describes a process for preparing Carbon filters using particles of size 35-200μ with 50% or more ceramic binder, in which water is sprayed onto this mixture before it is sintered.
To prevent and/or reduce the occurrence of bacterial colonization on the surface of the Carbon block, such blocks are generally impregnated with a metal such as Silver, Copper or Zinc.
U.S. Pat. No. 2,847,332 (Union Carbide Corporation, 1958) describes a process for impregnating metallic Silver onto a bound Carbon block in which a bound block is dipped into an aqueous solution of Silver nitrate and the Silver ions are precipitated as an insoluble Silver compound by exposing the dipped block to Ammonia fumes to precipitate Silver oxide. This is followed by reduction of the Silver oxide to metallic Silver by heat treatment. It is said that the process results in impregnated blocks with a uniform distribution of Silver content. This process is suitable for impregnation of higher levels of metals, but the present inventors have found that that blocks made in accordance with this process show variation in the amount of the impregnated metal across different blocks. This variation could become critical when the level of impregnation sought is lower.
US2006000763 (The Clorox Company, 2006), discloses a gravity flow carbon block filter comprising approximately 20-90 wt % activated carbon particles having a mean particle size in the range of approximately 90-220 mu·m, and approximately 10-50 wt % low melt index polymeric material. The low melt index polymeric material can have a melt index less than 1.0 g/10 min or greater than 1.0 g/10 min and a mean particle size in the range of approximately 20-150 mu·m.
U.S. Pat. No. 3,355,317 (Keith et. al., 1967), discloses impregnation of gas adsorbent materials with metal oxides for use in cigarette filters.
On the other hand, in the prior art process, there are two separate steps for block making; a metal impregnation step; and a block making step. The first process is generally done by the suppliers of activated Carbon particles. The two processes are independent of each other and therefore the cost of Carbon block manufacturers increases because the metal impregnated Carbon comes at a cost to the person making the blocks.
The present inventors have found that the above procedure is cumbersome as it is a two-step process. Further, this procedure is not suitable for impregnating lower levels of Silver or other metals, of the order of 0.01 to 5.0 wt %, while still showing relatively low level of variation in metal content across different blocks.